1. Field of the Invention
The present invention relates to a membrane electrode assembly for use in a solid polymer electrolyte fuel cell.
2. Description of the Related Art
Oil resources have been depleted, and at the same time, environmental problems including the global warming caused by fossil fuel consumption have been increasingly serious. Accordingly, fuel cells have attracted attention as clean electric power supplies for electric motors not involving the generation of carbon dioxide, and thus have been extensively developed and partially begin to be used practically. When the fuel cells are mounted in automobiles and the like, solid polymer electrolyte fuel cells using solid polymer electrolyte membranes are preferably used because such fuel cells can easily provide high voltage and large electric current.
Known as a membrane electrode assembly to be used in the solid polymer electrolyte fuel cell is a membrane electrode assembly which comprises a pair of electrode catalyst layers disposed respectively on both sides of a solid polymer electrolyte membrane having proton conductivity, and gas diffusion layers laminated respectively on the electrode catalyst layers. Each of the pair of the electrode catalyst layers is formed by supporting a catalyst such as platinum on a catalyst carrier such as carbon black and by integrating the supported catalyst into a single piece with an ion conducting polymer binder; one of the electrode catalyst layers acts as a cathode electrode catalyst layer and the other as an anode electrode catalyst layer. The gas diffusion layers are formed of, for example, carbon paper. The membrane electrode assembly may comprise two intermediate layers, each formed of a water-repellent resin containing electrically conducting particles, disposed respectively between one of the electrode catalyst layers and the gas diffusion layer pairing therewith and between the other of the electrode catalyst layers and the gas diffusion layer paring therewith. The membrane electrode assembly constitutes the solid polymer electrolyte fuel cell in combination with separators each doubling as a gas path and respectively being laminated on the gas diffusion layers.
In the solid polymer electrolyte fuel cell, the anode electrode catalyst layer is used as a fuel electrode into which a reductive gas such as hydrogen or methanol is introduced through the intermediary of the gas diffusion layer, and the cathode electrode catalyst layer is used as an oxygen electrode into which an oxidative gas such as air or oxygen is introduced through the intermediary of the gas diffusion layer. In this configuration, protons and electrons are generated in the anode electrode catalyst layer from the reductive gas by the action of the catalyst contained in the electrode catalyst layer, and the protons migrate to the electrode catalyst layer of the oxygen electrode side through the solid polymer electrolyte membrane. The protons react with the oxidative gas and the electrons introduced into the oxygen electrode to generate water in the cathode electrode catalyst layer by the action of the catalyst contained in the electrode catalyst layer. Consequently, connection of the anode electrode catalyst layer and the cathode electrode catalyst layer with a conductive wire makes it possible to form a circuit to transport the electrons generated in the anode electrode catalyst layer to the cathode electrode catalyst layer and to take out electric current.
In the membrane electrode assembly, the protons migrate along with water in the solid polymer electrolyte membrane. Accordingly, the solid polymer electrolyte membrane needs to have appropriate moisture. Such moisture is supplied, for example, by the reductive gas or the oxidative gas. However, there is a problem in that no sufficient electric power generation performance can be attained when the humidity of the reductive gas or the oxidative gas is low.
On the other hand, as described above, in the membrane electrode assembly, the electric power generation is accompanied by the generation of water in the cathode electrode catalyst layer. Consequently, a long-time operation of the fuel cell makes excessive the moisture in the membrane electrode assembly to inhibit the diffusion of the reductive gas or the oxidative gas, and hence this case also suffers from a problem that no sufficient electric power generation performance can be attained.
Various proposals have been presented to overcome the above described problems, including, for example, a technique in which in a membrane electrode assembly for use in a solid polymer electrolyte fuel cell comprising a pair of electrode catalyst layers disposed on both sides of a solid polymer electrolyte membrane and gas diffusion layers laminated respectively on the electrode catalyst layers, the pores in the gas diffusion layers are regulated. In this technique, the porosity of the gas diffusion layers is specified to fall within a range from 45 to 75%, and the specific volume of the pores, in the gas diffusion layers, falling within a pore size range from 17 to 90 μm is specified to fall within a range from 0.45 to 1.25 cm2/g (see Japanese Patent Laid-Open No. 11-144740).
Also known is a technique in which in a membrane electrode assembly, for use in a solid polymer electrolyte fuel cell, comprising a gas diffusion layer laminated on each of the electrode catalyst layers through the intermediary of an intermediate layer, wherein the pores in the electrode catalyst layers are specified. In this technique, the total volume of the pores in the electrode catalyst layers falling within a pore size range from 0.01 to 30 μm is specified to be 6.0 μl/cm2 or more per 1 mg of the catalyst (see Japanese Patent Laid-Open No. 2004-193106).
Also known is a technique in which in a membrane electrode assembly, for use in a solid polymer electrolyte fuel cell, comprising a gas diffusion layer laminated on each of the electrode catalyst layers through the intermediary of an intermediate layer, wherein the pores in the gas diffusion layers are regulated. In this technique, the density of each of the gas diffusion layers is specified to fall within a range from 0.2 to 0.55 g/cm3 and the peak pore size in the pore size distribution in each of the gas diffusion layers is specified to be 10 to 100 μm (see Japanese Patent Laid-Open No. 2004-296176).
Further known is a technique in which in a membrane electrode assembly, for use in a solid polymer electrolyte fuel cell, comprising a gas diffusion layer laminated on each of the electrode catalyst layers through the intermediary of an intermediate layer, wherein the pores in the intermediate layers are regulated. In this technique, each of the intermediate layers is constituted with at least two layers different in pore size from each other, wherein the pore size distribution in each of the intermediate layers is made to have a gradient in such way that the pore size of the layer, of the at least two layers, on the electrode catalyst side is made to be smaller than the pore size of the layer, of the at least two layers, on the gas diffusion layer side (see Japanese Patent Laid-Open No. 2001-189155).
However, any of the above described conventional techniques involves disadvantages such that the pore size made smaller leads to insufficient gas diffusivity and insufficient capability of discharging the generated water, and the pore size made larger leads to insufficient water retentivity.